COMPATIBILTY PRE-SCREENING OF BENTONITE-BASED BARRIER SYSTEMS

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Abstract

Soil hydraulic conductivity is related to its internal structure, which is in turn a function of its mineralogy and placement conditions. However, this structure is not constant and changes with time as a result of chemical, physical and biological processes. Commonly, these changes result in a net increase in hydraulic conductivity.
A laboratory testing program was established to evaluate the performance and characteristics of two different bentonites. Index tests can be used to evaluate the performance of bentonite when exposed to chemical wastes. Some index tests are better than others. According to this research the author suggests that the most helpful and reliable index tests are fluid loss and plastic viscosity.
The filter press test was extended to yield an estimate of the hydraulic conductivity of the resulting filter cake. Upon comparing the relative hydraulic conductivity values obtained from the filter press and the values obtained from permeameters, similar trends were observed with DDL thickness contraction;as the DDL thickness contracts the hydraulic conductivity increases.
Anew alternative pre-screening method using plastic viscosity and the filter press was outlined and compared to the existing ASTM 0 6141 free swell method. The plastic viscosity method consistently reported more conservative and reliable results that have been verified with hydraulic conductivity testing. A compatibility plot with two warning limits has been identified for the two bentonites exposed to electrolyte solutions. The first limit identifies when slight increases in the hydraulic conductivity are expected and further testing is required to determine the extent of the increase. The second limit identifies when significant increases in the hydraulic
conductivity will occur and that a different liner material should be used.
A high pH, phenolic wastewater was obtained and the two pre-screening methods were used to assess the chemical compatibility of the wastewater with two different bentonite products. The plastic viscosity method returned consistently accurate results, for both bentonites, which were verified with hydraulic conductivity testing, whereas, the ASTM D 614] method returned two different results for the two bentonites used. It was concluded that the high pH, phenolic
wastewater would slightly increase the hydraulic conductivity of a bentonite-based barrier system.
The author considers the new plastic viscosity method to be better than the existing pre-screening method outlined in ASTM 0 6141. Although shown to have faults, the use of ASTM D 614] may still be continued,as it is in expensive and easy to perform in any laboratory with a graduated cylinder. The plastic viscosity method requires a device to measure the viscosity of a soil slurry; therefore, testing is limited to laboratories that possess such a device.